Fix menu item width when the delegate is overriden

Summary:
Have MenuItem get the width of the parent instead of needing to specify it
every time.
This fixes an issue where MenuItem is used from Kirigami.ActionsMenuItem where
it would sometimes not take the whole width. See bug.

BUG: 401792

Reviewers: #kirigami, mart

Reviewed By: #kirigami, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17484